The present invention relates generally to radar cross-section (RCS) and antenna pattern measurement ranges. More specifically, the present invention relates to low RCS support/sting pylons for use with low RCS and antenna pattern measurement ranges. The low RCS support/sting pylon are particularly advantageous when employed with a geometrically shaped measurement chamber, which removes substantially all but direct path backscattered signals from the target of interest. The present invention also relates to high performance anechoic chambers for antenna pattern measurement. A low RCS measurement system which exploits the novel low RCS support/sting pylons is also disclosed.
A radar system tracks a target in response to an echo, i.e., a reflected portion of the incident radar signal, from the target of interest. Therefore, it is critical to the design and operation of radar systems to be able to quantify, or otherwise describe this echo, particularly in terms of target characteristics, e.g., size, shape and/or orientation. One such characteristic is radar cross-section (RCS), which is the projected area of a metal sphere returning the same echo signal as the target of interest, assuming the metal sphere is substituted for the target of interest. Unlike the echo signal from a sphere, which is orientation independent, the echo signal, and thus the RCS, varies as a function of orientation of the target of interest. This variation can be very rapid, especially when the target of interest is many wavelengths in size.
RCS values of simple bodies can be computed exactly by solution of a wave equation defined in a coordinate system for which a constant coordinate coincides with the surface of the body. However, there is no known tactical target of interest which fits these solutions. The practical engineer cannot rely on predictions and calculations; the engineer must eventually measure the echo characteristics of the target of interest. This measurement can be performed on a full scale target of interest on an outdoor test range or on scale models to the target of interest in a measurement chamber. Current state of the art ranges include "compact ranges" which use a collimating reflector system to achieve the desired electromagnetic field distribution in the measurement zone (target area), i.e., to simulate a wide separation between the radar source and the target of interest.
Typical RCS chambers are rectangular rooms covered with Radar Absorber Materials (RAM). For a given target support system and antenna/radar system, the chamber performance is limited by the chamber size and shape and by absorber material employed. Cost limitations usually drive both the chamber size and the quantity and quality of the RAM installed in the chamber. The measurement capability in RCS chambers is limited by several factors, including:
(a) the chamber size and shape;
(b) the type and amount of RAM applied to the chamber walls;
(c) the target support system; and
(d) the antenna/radar system.
With respect to factors (a) and (b), the room, i.e., measurement chamber itself, is often the limiting factor in RCS measurement chambers. Radar reflections or echo signals are generated by scattering from the target of interest. Echo signals which are not direct path generally arrive at a later that the direct path backscattered signals and, thus, can contaminate the RCS measurement. The conventional method of "quieting" the radar reflections from the room itself is by treating the chamber walls with large pyramidal RAM up to six feet deep that attenuates the incident microwave energy. With respect to factor (d), radar range (time) "gating" (either true short pulse or synthetic short pulse) may be used to remove most radar scattered signals that arrive at the radar at a time other than the desired return from the target. However, since RAM provides only limited attenuation and since radar range (time) gating cannot provide echo signal cancellation to completely eliminate unknown short bounce interaction, the RCS measurement is usually contaminated by spurious echo signals. In other words, some diffuse returns from the absorber as well as some chamber wall returns will arrive at the radar at the same time as the desired target backscattered and cannot be gated out. In particular, surfaces perpendicular to the chamber long axis, such as those associated with conventional target and source supports, are especially prone to causing spurious or unwanted scattered signals. These scattered signals establish the background levels of the chamber. Since the target should be at least 10 decibels (dB.sub.SM) above the background level, this background level also establishes the limit on the lowest RCS target which can be measured using a given range.
With respect to factor (c), attempts have been made over the years to reduce the spurious scattered signals which contaminate RCS measurement, particularly those generated by the support for the target of interest, since the target support pylon is one of the main factors which limits the measurement capability of existing RCS measurement ranges. The three different kinds of support structures which are generally used in RCS measurement schemes are discussed immediately below.
(1) Low Density Plastic Foam Columns. U.S. Pat. No. 5,099,244 discloses a typical example of such a pylon. Such columns are not echo free. The echo from the plastic foam column is generated by two separate mechanisms, coherent surface reflections and a non-coherent volume contribution from the thousands of cells comprising the foam material. It will be appreciated that in order to minimize the RCS of the foam column, the pylon must use low dielectric constant materials which have a low density and, thus, have severe limits on the target weight which can be supported. One attempt to overcome the load limitation of foam columns was proposed in U.S. Pat. No. 5,028,928, which discloses a pylon formed from an inflatable stressed skin bladder having a low RCS value. It will also be noted that the physical attachment of the target to the foam limits the range of target elevation angles which can be measured. In addition, the RCS from the foam material increases at higher microwave frequencies, which limits the measurement capability, particularly with respect to scale model targets.
(2) String Suspension Harnesses. U.S. Pat. No. 5,075,681 discloses a cable support system of this type. The echo signal from a string depends on the length and diameter of the string, its tilt angle with respect to the incident wave, and its dielectric constant.
(3) Slender Metal Pylons. U.S. Pat. No. 4,990,923 discloses a pylon which includes an elongated support extending upwardly from a base to a tip on which the target of interest is mounted. The support inclines and curves in the direction of the radar source. The tip of the support, which is formed from RAM, is smoothly faired to the target. A typical metal pylon owes its electromagnetic performance to the sharpness of the leading edge and its tilt toward the radar source. RAM covered pylons are generally metallic structures of ogival cross section. These pylons have a leading edge, i.e., the edge facing the radar, which is sloped toward the radar at approximately 28.degree. from vertical, i.e., perpendicular to the long axis of the low RCS measurement chamber. It should be mentioned that target elevation angles for this type of pylon are limited to "look down" angles only and viewing angles are limited by the target-to-pylon interface geometry. Radar Absorbing Structure (RAS) pylons of ogival cross section, or other more complex cross section, such as that disclosed in U.S. Pat. No. 4,809,003, for lower RCS levels, are similar to the RAM covered pylon described above. These pylons suffer the same target-to-pylon limitations due to the size of the top of the pylon.
A variation on this last configuration is disclosed in U.S. Pat. No. 4,713,667, wherein two vertical circular columns which are movable with respect to one another are provided to support the target of interest, and wherein the columns can be positioned so that radar reflections from the two columns will cancel one another.
What is needed is a low RCS target support/sting pylon which can be installed in a RCS measurement facility and which provides a target support structure with an extremely low RCS signature. Advantageously, the low RCS support/sting pylon produces minimal scattered signals, since the "sting pylon" has a minimal profile perpendicular to the long axis of the low RCS measurement chamber, i.e., since the radar is "looking" down the length of the horizontal target support pylon. Moreover, it would be beneficial if the low RCS target support/sting pylon provided a support structure wherein the target-to-pylon interface remains constant as the target is tilted in elevation. What is also needed is a low RCS support/sting pylon wherein the elevation tilt axis is behind the target and wherein the elevation joint, e.g., pivot element, is covered by an elastomeric conductive sheet, so as to provide a relatively low RCS contribution due to the elevation joint. The unwanted scattered signals advantageously could be further reduced by a low RCS source support/sting pylon which can be installed in a RCS measurement facility and which directs scattered signals out of the RCS measurement chamber.